1. Field of the Invention
The present invention relates to a superimposing technique for use in television production switchers for superimposing text or the like on background images. The present invention is particularly concerned with a video processor for producing video and key signals to be used in television production switchers.
2. Description of the Related Art
Television production switchers are currently provided with memories known a video store, a key store and a frame store for storing video signals and key signals for producing superimposed images using the stored video signals. These memories are provided separately from a video processor. As shown in FIG. 1, a prior art video processor is comprised of two field-delay memories designated 102 and 103. Input video signal is used in two ways, one being directly coupled to the field-delay memory 102 and the other to a superimpose key signal generator 101 to which an input key signal is also supplied. Key generator 101 is responsive to a select signal for selecting one of its input signals and shapes the waveform of the selected signal to eliminate noise. Field-delay memory 103 is connected to the output of the key generator 101. The output of each field-delay memory thus represents the signal that is one-field earlier than its current input signal. The effect of this delay is to reposition the video and key signals to their original scanning line. The output signals of field-delay memories 102 and 103 are coupled to a known shadow inserter 104.
Since the field-memories are used exclusively for repositioning the video and key signals, read/write control is not possible, and hence the contents of these memories cannot be used for superimposing during subsequent frames.
It is therefore an object of the present invention to provide a latch circuit with a reduced number of circuit nodes and a flip-flop implemented with the latch circuit.
According to the present invention, there is provided a video processor for producing video and key signals to be processed by a television production switcher for superimposing text or the like on a background image, the video processor comprising a key generator for receiving an input video signal and an input key signal and selecting one of the input video and key signals and shaping waveforms of the selected signal and producing an output signal, a first frame memory for receiving the input video signal and storing the received signal and reading the stored video signal according to a first control signal applied thereto, and a second frame memory for receiving the output signal of the key generator and storing the received signal and reading the stored signal according to a second control signal applied thereto.
The video processor further includes a delay circuit for introducing a delay time corresponding to processing time of the key generator to the input video signal, a first switch for selecting one of an output signal of the delay circuit and an output signal of the first frame memory, and a second switch for selecting one of the output signal of the key generator and an output signal of the second frame memory. A shadow insertion circuit is provided for receiving output signals of the first and second switches for inserting a shadow to the received signals.